1. Field of the Invention
Aspects of the present invention relate to an electrode assembly and a secondary battery having the same, and more particularly, to a secondary battery capable of ensuring optimal battery safety without diminished performance.
2. Description of the Related Art
In recent years, the rapid development of small and lightweight portable electronic devices has generated an increasing need for high-capacity, small-sized batteries. In particular, lithium ion secondary batteries can provide an operating voltage of at least about 3.6 V, which is about 3 times higher than nickel-cadmium batteries or nickel-hydrogen batteries widely used in portable electronic devices, and lithium ion secondary batteries have a higher energy density per unit weight than nickel-cadmium batteries or nickel-hydrogen batteries. For these reasons, research into lithium ion secondary batteries has rapidly progressed.
In a lithium ion secondary battery, electrical energy is generated due to oxidation and reduction reactions, which occur when lithium ions are intercalated/deintercalated at positive and negative electrodes. Fabrication of the lithium ion secondary battery involves forming positive and negative electrodes out of materials capable of reversibly intercalating/deintercalating lithium ions, and filling an organic electrolyte or polymer electrolyte between the positive and negative electrodes.
The lithium ion secondary battery includes an electrode assembly in which a negative electrode plate and a positive electrode plate, with a separator interposed therebetween, are wound in the form of a jelly-roll, a can that contains the electrode assembly and an electrolyte, and a cap assembly mounted on the can.
Conventionally, for the separator, a single or multiple polyolefin micro-porous polymer layer formed of at least one of polypropylene (PP) and polyethylene (PE) has been used. However, since a polyolefin micro-porous polymer layer that serves as a separator has a sheet or film shape, when heat is generated in a battery due to internal shorting or overcharge, pores may be clogged and the film-type separator may shrink.
Accordingly, when the film-type separator shrinks due to heat generated in the lithium ion secondary battery, portions of the positive and negative electrodes, which are not separated by the shrunk separator, are brought into contact with each other, thereby causing ignition, bursting, or explosion.
In order to overcome these disadvantages of the film-type separator, a considerable amount of research has focused on forming a ceramic separator formed by coating a film-type separator with a porous layer comprising a binder and a ceramic material, such as silica (SiO2), alumina (Al2O3), zirconium oxide (ZrO2), or titanium oxide (TiO2).
In this case, the ceramic separator may make up for the fusion and shrinkage of a film-type polyolefin separator at a high temperature of about 120° C. or higher. As a result, there is a growing tendency to use both a conventional film-type separator and a ceramic separator.
Generally, the performance of a battery is evaluated in terms of safety and reliability. Charge and discharge cycle characteristics are considered to be the most important factors affecting the reliability of the battery. The length of time that a mobile device can be stably used without interruption of power depends on its capacitance maintenance rate with respect to the number of charge and discharge cycles it has undergone.
While capacitance is generally evaluated based on charge and discharge cycles at room temperature, in order to reproduce extreme conditions, cycle characteristics may be evaluated at a low temperature of 0° C. to 10° C.
However, when a conventional film-like separator is combined with a ceramic separator in a battery, cycle characteristics at low temperature deteriorate.
That is, when a film-like separator is combined with a ceramic separator, battery safety is enhanced but reliability deteriorates.
Therefore, matching a film-like separator with a ceramic separator is important, and it is desirable to optimally design a lithium ion secondary battery so as to ensure its safety without diminishing its reliability.